LED'S USED AS RECTIFIER DIODES.....

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Janitor Tzap
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LED'S USED AS RECTIFIER DIODES.....

Post by Janitor Tzap » Sat Oct 14, 2006 4:37 pm

Thank's with the help on the 6V Power Supply everyone.

But, here's a question for the Engineer's here in this forum.

I saw back in the early 80's, at this local Stereo/Television dealer.
They were selling "Luxman" Audio Equipment.
A pamphlet that was being given out to those who were interested in it.
It discribed a fullwave bridge rectifier that was made with LED's,
that was used in they're amplifiers.
Unfortunately, none of the display models were opened up so you could see inside.

So, has anyone seen one of these amplifiers?
And doe's it really used LED'S for rectifier diodes?

I'm tempted too see if it can be done.
I've got a bunch of LED'S just laying around.
So, I'm thinking about trying it.
But, I would like to get some input from you all first.

Thanks.

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Timothy Rasch
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leds as rectifiers

Post by Timothy Rasch » Sat Oct 14, 2006 6:47 pm

Hi, I have seen in GE portable mixers the charging circuit used the red led as rectifier in series with a resistor [vaguely remember value 180ohms or lower and a zener in series with it so it would stop charging when the battery was full.]. The leds can be used in a bridge as a LOW current rectifier only because if a high current is passed through them they will burn out. In the amp you talk about I think they used the led bridge as output signal limiter like Hammond Organ used two #12 light bulbs and two resistors 68ohms,100 ohms in a wheatstone bridge configuration from the output of the reverb drive amp to reverb transducer input [Hammond A0-44 in model A100 organ].There is a free schematic site for Hammond Organ [Google it] .This was done by Hammond to protect the reverb transducer from damage in case there was too much drive from the amp. In your amp that you talked about I think the leds bridge was for speaker protection just like the Hammond reverb transducer protection with a modern approach instead of bulbs. Tim Rasch [email protected]

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MrAl
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Post by MrAl » Sat Oct 14, 2006 7:30 pm

Hi there,

As already mentioned you can use LEDs for rectifiers but they have
certain limitations that regular rectifiers dont.

First, as mentioned, the current will be very low compared to modern
Si diodes because the LEDs cant handle much current. This means
you can only use them for a low power power supply.

Second, they have a huge voltage drop compared to Si diodes.
This means your power supply will be much less efficiency than when
using regular diodes.

Third, LEDs can not take too high of a reverse voltage, meaning
either you can only use them for very low voltage supplies (less
than 5vdc) or you have to include a diode and resistor to protect
each LED. Hardly seems worth it as the LED and diode will be
dropping the voltage of the LED plus the voltage of the diode it
was supposed to be replacing.
LEDs vs Bulbs, LEDs are winning.

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Post by Robert Reed » Sat Oct 14, 2006 9:25 pm

Jan
If the LED's were not visible from out side the case, what purpose would they serve? As MrAl pointed out, they would be a terrible choice for a power supply rectifier. The PIV is the comination of the dc voltage on one side of the diode and the peak opposite excursion of the AC on the other side of the diode.For most the LED specs I have seen this would limit its use to VERY low voltage supplys. Add to that the fact that it would be dropping about 2 volts of that meager DC across it (even more if you use a bridge configuration), and the resultant output is now lower yet. In signal flow applications,it would be useless as to the severe distortion it would introduce. Are you sure you read the pamphlet right, as it makes no sense at all. I realize its true that one can learn something every day, but I have looked at this a dozen different ways and still can't come up with a practical use of this.

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Post by Edd » Sun Oct 15, 2006 2:11 am

.

Re:
http://www.audioinvest.no/luxman/products/l_m02.htm

Image


LED Bias Current:
Luxman's unique bias circuit employing LED's instead of diodes provides speedy attack and decay in actual auditory feelings.


1...(They are only used in the crossover distortion biasing level of the output stages of the power out stage ...thus only being in the sub or decade of low ma's)
.......But you can watch them glow if viewed
2...Hmmmmmmm, now isn't that.... "actual auditory feelings"... impinging upon the touchy touchy- feely feely semantics of an "audiophool" ?


High-efficiency, liquid sealed-tube type heatsink used even for artificial satellite.

(e.g.....Heat-pipes)

73's de Edd
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Post by dyarker » Sun Oct 15, 2006 3:44 am

Why not? Red or amber LEDs would have about the right forward voltage to bias Darlington output transistors.
Dale Y

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Post by Chris Smith » Sun Oct 15, 2006 6:02 am

I used to make battery trickle chargers in the 80s out of Leds.

Their current was low, the lights told everything and they work just fine as long as you don’t over stress them.

For a greater current, you need to by pass some of the heavy current.

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Post by Janitor Tzap » Sun Oct 15, 2006 1:18 pm

Edd wrote:.

Re:
http://www.audioinvest.no/luxman/products/l_m02.htm

Image


LED Bias Current:
Luxman's unique bias circuit employing LED's instead of diodes provides speedy attack and decay in actual auditory feelings.


1...(They are only used in the crossover distortion biasing level of the output stages of the power out stage ...thus only being in the sub or decade of low ma's)
.......But you can watch them glow if viewed
2...Hmmmmmmm, now isn't that.... "actual auditory feelings"... impinging upon the touchy touchy- feely feely semantics of an "audiophool" ?


High-efficiency, liquid sealed-tube type heatsink used even for artificial satellite.

(e.g.....Heat-pipes)
Thanks Edd,

I think that's the one I saw at that store I was at.
{It's been over 20 years. So I can't be completely certain.}

I've never worked on a Luxman.
I know many of the audio amplifiers of that time period,
were trying all kinds of different pre-amp designs to get the cleanest
hiss free signal possible. Before going to the Main amplifier.
While others were trying to get as much hiss out of the Main amplifier.
And some even did both.

Thus, I'm not that surprised that their using the LED's in the bias circuit.

As for LED'S used for rectifing AC.
I haven't come across any actual circuits or schematics that show LED's in them.


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Post by Robert Reed » Sun Oct 15, 2006 4:56 pm

LED Bias Current:
Luxman's unique bias circuit employing LED's instead of diodes provides speedy attack and decay in actual auditory feelings.

Attack and decay what? I have only seen these terms when describing Companders as used in audio systems. Does any one know what they are signifying here, and what advantage would LEDs play in it?

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MrAl
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Post by MrAl » Sun Oct 15, 2006 11:45 pm

Hi there,

Sounds like they are talking about using the LED in place of a diode
where the diode's bias is changed in order to increase or decrease
the "ac resistance" of the device so it can be used as a voltage
controlled resistor for low level audio signals. This would be used
as part of an agc (automatic gain control) circuit.

The idea is to bias the diode with a dc current and capacitively couple
the low level ac signal across the diode through a fixed impedance
source. The result is as the dc current is changed the ac signal output
(also cap coupled to the output stage) increases or decreases, depending
on the amount of dc current.

The idea of using an LED instead of a diode for this doesnt really seem
like any significant technological advance however, but it might give
a greater change of gain with current which would mean less current
might be needed to operate it. I'd have to do some tests to see if
this is true or it's just a total gimick. Perhaps it was just a selling point
as offhand i cant think of any reason why a 2v diode would work any
better than a 0.7v diode. It certainly would *not* be *faster*, because
the LED has more junction capacitance than ss Si diodes so if anything
it would be *slower*, but compared to audio they are both fast.
LEDs vs Bulbs, LEDs are winning.

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Post by Edd » Mon Oct 16, 2006 1:31 am

.

I thought that my topic 2 touched adequately upon the language disparity involved.
With the use of the language comparisons to acoustical performances as might be described by a “discriminating audiophoolâ€

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MrAl
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Post by MrAl » Mon Oct 16, 2006 2:02 am

Hi there Edd,

Edd wrote: And, as for any hint of the design of compressing the acoustical high and low thresholds of an amps range. NO WAY…..one would want the acoustical extremes to adequately encompass the winky-tinking of a mouse upon a cotton ball on the low side on up to the extremes of being fuselage side to the propwash of a Huey Cobra on the hi extreme.
Well, that's not always true. It's true for music, but not for speech.
When an amp with a mic is used for someone speaking to a large
crowd sometimes it's very good to have compression, which i assume
a good amp would want to have. Of course it would be switchable
in/out as a good amp would have.
LEDs vs Bulbs, LEDs are winning.

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Post by Dean Huster » Mon Oct 16, 2006 5:59 am

Tektronix used an LED for biasing in their 1502/3 TDR as I recall, the diode inside the waterproof case where it would never be seen. When asked about it, the standard answer was always, "It's a light for the tiny little engineer inside ....."
Attack and decay what?
Isn't that otherwise known as a termite infestation?
When an amp with a mic is used for someone speaking to a large
crowd sometimes it's very good to have compression, which i assume
a good amp would want to have.
Yeahbut .... isn't that why we're always hammered with the commercials when the volume is at a low level for the regular program? Most folks I've experienced using PA systems indoor or out are only concerned with how much bass they can pump out.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Chris Smith
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Post by Chris Smith » Mon Oct 16, 2006 9:02 am

You underestimate the sales pitch ideas of the old days.

Leds were coming down in price, a perfect reason to use them up?

Then they were also thought of as technologically fancy, a great optical sales pitch.

A lot of them sounded good, but most of them were just fluff for the sales angle.

Almost as bad as the Led VU meters, like you need to visualize what your ears already tell you?

But it looks good?

Leds do have certain properties but so do many other related diodes, but they don’t shine simply because they are encapsulated in dark epoxy.

Cost was probably the big key reason.

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Post by haklesup » Mon Oct 16, 2006 11:14 am

An LED can be used in any circuit requiring a Diode with a characteristic to match. That said,other than the obvious lighting application, I have only seen them used to bias various bipolar transistor circuits.

The marketing guys were probably trying to capitalize on the still strong tube amplifier crowd. I can just hear the strategy meeting now....."if we have something that glows, it will remind them of tubes and they will accept solid state better". There may have been a good reason to use it in the design but there really is no technical reason to advertize that fact.

BTW: While Si junctions (diodes) do emit light but it is very dim and even if packaged in clear envelopes (1N914 for example) you would not see anything without a very sensitive camera and a dark box. In fact a technique called Emmission Microscopy looks for defects in IC devices by looking for anomalous light coming from the die. Reverse biased junctions are usually dark unless damaged.

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